The development of photographic recording systems has seen the introduction of more precise data transmission and reproduction equipment such as laser recording devices, using acoustic-optic modulator components. An example of such a laser facsimile recorder is described in the U.S. Pat. No. 3,997,722 to Bardos. In this type of system, information to be recorded in the form of a photographic image is modulated onto a recording laser beam as the beam is scanned across a moving photographic recording medium, such as sensitized paper or film. The overall photographic image is comprised of a set of substantially parallel lines the widths of which vary as the intensity of beam is modulated during its sweep across the film. The incidence of the beam on the film is controlled by a mirror galvanometer arrangement that deflects the modulated beam to form each line scan or trace across the film as the film is driven by the film drive system.
For the purpose of driving the film during the recording of each line trace, stepping motors have been found to be particularly useful as their control is readily compatible with signals produced by digital processing circuitry that is customarily employed in present day data processing and recording systems. During the recording of each line or trace across a prescribed width of the film that defines the width of the image to be formed, the stepping motor is clocked or advanced a plurality of times, to thereby form a stepwise linear trace or line. Although this type of film advance arrangement is preferred because of its system compatibility and precise film advance control, it has suffered from the drawback of introducing into the recorded image one or a series of imperfections called bands.
More specifically, and with reference to FIG. 1 of the drawings, there are shown a plurality of lines, such as those formed by a conventional laser facsimile system described in the above referenced patent, that form part of the overall recording image. (Only four lines are shown in order to simplify the illustration and to facilitate its description). As shown in FIG. 1, the X or horizontal direction represents the direction in which the recording optical beam, e.g. a galvo-mirror deflected line beam, is moved, while the vertical or Y direction corresponds to the direction in which the photographic recording medium is advanced by the stepping motor.
Looking at trace 1, for example, as the recording beam is deflected across the surface of the recording medium, the film is advanced incrementally, at beam deflection points 1a, 1b, 1c, 1d by the stepping drive motor, so that there is recorded a stepwise diagonal line trace across the width of the recording medium. Subsequent lines or traces 2, 3 and 4 are shown as being incrementally advanced at deflection points 2a, 2b-3a, 3b-etc. as each new line is begun and the beam is deflected as the recording medium is stepwise advanced. Due to lack of precise synchronization, the points at which successive lines are stepwise-advanced by the stepping motor are offset in the direction of deflection of the galvoscanning mirror arrangement, so that there is created a composite series of bands which extend diagonally across the paper and which constitute degradation of the image copy. Ideally, the points at which the film or paper is advanced (which points are, in reality, smoothed somewhat by the behavior of the film advance motor which does not actually produce sharp transition in the recorded lines) should be the same for each successive line, so that these points are aligned in the vertical or Y direction, as opposed to being diagonally offset as shown in FIG. 1. Efforts to reduce banding have included the use of very precise gear train arrangements between the drive motor and the recording medium roller mechanism. Unfortunately, the tolerances required to provide the necessary synchronization accuracy make this approach prohibitively expensive.
In addition, with conventional systems, synchronous control of the stepped advance of the recording medium for changes in resolution of the recorded image without suffering a marked distortion or boundary within the image has not heretofore been achieved.